1. Field of the Invention
The present invention relates to a half-duplex digital transmission system having two terminal stations linked by a two-wire line including bidirectional repeaters.
2. Description of the Prior Art
Half-duplex digital transmission systems are disclosed in the German Patent Application No. 2,040,401 and Canadian Pat. No. 1,081,816 and the article by P. Hirschmann and K. Wintzer entitled "Design concept and features of digital subscriber sets", published in "International Zurich Seminar on digital communications", pages D1.1 to D2.4, Zurich, Switzerland, Mar. 7-9, 1978.
Generally speaking, one of the terminal stations is a subscriber digital line concentrator within a telephone switching exchange and the other terminal station is a subscriber terminal such as a digital telephone set.
An encoding and decoding device associated with the telephone set simultaneously emits and receives digital signals which correspond to the encoding of voice signals. The resulting bit rate is for instance equal to 64 kbits/s. This corresponds to PCM encoding a voice signal sample every 125 .mu.s into an octet.
The half-duplex transmission imposes a digital bit rate in line at least equal to twice the initial one of 64 kbits/s. In other words, each two-wire line conveys packets having at least eight bits along both transmission directions alternately such that each packet occupies a time interval which is less than 125/2 .mu.s. The line bit rate is equal, for example, to 256 kbits/s. As a result, over one half-duplex transmission cycle, which is defined by the temporal difference between the emission of two successive packets from one terminal station, each station emits only one packet and receives only another packet from the remote station.
In accordance with the prior art, a packet is transmitted directly from one station along a two-wire line to the other remote station. During one transmission cycle, a first packet is emitted, in one of the transmission directions, from a first of the stations and received by a second station; then a second packet is transmitted from the second station having just received the first packet and is received by the first station. Consequently, in the repeaters inserted on the two-wire line, all the amplifying means related to the first packet transmission direction are connected to the two line sections adjacent to the repeater; then the amplifying means related to the opposite, second packet transmission direction are connected to the two adjacent line sections and so on.
Due to this, for a given bit rate and a given format of the packets conveyed along the line, the range of the two-wire line, i.e. its maximum length is limited by the half-duplex transmission cycle duration imposed by the "master" station, in this case the line concentrator. Put another way, this means far-flung subscriber terminals cannot be linked up to the concentrator via very long digital two-wire lines.